From Caspar David Friedrich and the Subject of Landscape by Joseph Leo Koerner:
A devoutly Protestant painter, Friedrich was a distant heir to the iconoclasts of the Reformation…His endeavor to void his canvas of all subjects except one—the believer or Christian subject vis-à-vis the hidden object of belief—obeys this pious imperative to negate. Friedrich’s Protestantism is more confessionally specific than this, however. His effort to create landscape painting as a new kind of religious icon, one resolutely in and of the secular world yet reaching beyond, transcendently, derives from the specifically Lutheran settlement on images. Although Luther repudiated church pictures as instruments of salvation, he condemned—more vehemently—violent iconoclasts, observing that their fanatic war against images made them the idolaters.